Superape146;s genescape

On a practical level, researchers said, the work is likely to explain why chimps are resistant to several human diseases such as AIDS, hepatitis, malaria and Alzheimer8217;s disease 8212; information that could lead to new ways to prevent or treat many human ills.

More profoundly, however, the achievement promises to help answer the alluring but loaded question of what, exactly, makes us truly human.

But the answer will not come easily.

8220;We8217;re not going to stand up and say that these 14 things make us human,8221; said Eric Lander of the Broad Institute in Cambridge, a facility run by the Massachusetts Institute of Technology and Harvard, which along with Washington University in St. Louis led the chimpanzee genome sequencing effort. 8220;But it8217;s not trivial to be able to say, 8216;Here is an inventory of the most important differences, and now go at it and figure out which of these differences contain the signatures of what is distinctively human.8217;8221;

As predicted by preliminary studies, the human and chimpanzee genetic codes are essentially 99 per cent identical, a testament to how fundamentally similar the two species remain. At the same time, it is powerful evidence that seemingly modest changes in molecular code can lead to very different stations in the web of life.

Because of that 1 per cent difference, experts noted, humans now dominate every ecosystem on earth while chimpanzees and other great apes 8212; a group that also includes bonobos, gorillas and orangutans 8212; are at risk of becoming extinct within the next few decades, largely because of human activities.

Well aware of that awkward reality, several scientists used the occasion of the chimp genome8217;s unveiling to focus attention on the creatures8217; plight, calling for renewed conservation efforts and new rules governing the use of great apes in research.

8220;There is a deep irony in the fact that the sequencing of the chimpanzee genome coincides with the potential demise of great apes in the wild,8221; wrote Ajit Varki of the University of California at San Diego, and colleagues, in a commentary accompanying the main research report in Nature.

The DNA analysis 8212; the first of a non-human primate and the fourth of a mammal after human, mouse and rat 8212; was done on blood drawn from a chimp named Clint, who lived at a research center in Atlanta. Key scientific findings and related commentaries fill about 100 pages in Nature and Science.

The human and chimp genomes are distinguished by 35 million differences in individual DNA 8220;letters8221; 8212; each the result of a tiny, random mutation 8212; and another 5 million larger differences in which entire chunks of DNA were either added to or deleted from one genome or the other.

All told, the two sequences differ by 4 per cent. But three-quarters of the differences seem to be in nonfunctional parts of the genome, suggesting that a mere 1 percent variation makes all the difference.

Put another way, the difference between the human and chimp genomes is 10 times as great as the difference between any two humans.

Among the genes that appear unique to humans are some involved in brain development and body plan and one that has been postulated as being crucial to the development of language. But most of the differences between chimpanzees and humans seem attributable not so much to the genes themselves but to how genes that both species share are regulated 8212; that is, the timing and level of intensity under which those shared genes operate.

8220;The class of genes that has changed the fastest in humans compared to chimps are the genes that control other genes,8221; said Tarjei Mikkelsen of the Broad Institute.

Developmental changes are behind many of the differences between human and chimp brains. Human brain cells divide several more times than chimp brain cells during fetal development, a fact that contributes to the adult human brain8217;s growth to three times the size of the chimpanzee8217;s. Much of that increase is in the cerebral cortex, home to higher cognition.

But scientists confess to knowing little about how such changes might add up to differences in intellect and behavior.

8220;We are woefully ignorant about how genes build brains, and how the electrical activity of the brain builds thoughts and emotions,8221; wrote Marc Hauser, co-director of Harvard8217;s Mind, Brain and Behavior Program.

Chimpanzees have repeatedly toppled conceptions about the ways in which humans are purportedly unique.

In perhaps their cheekiest aping of humanity, chimpanzees display remarkable political acumen. They form complex alliances and trade grooming services, sex and food. Like many denizens of the world8217;s great cities, they lobby, demand bribes, repay favors and, when crossed, extract revenge.

8220;I can8217;t imagine Darwin hoping for a stronger confirmation of his ideas,8221; said Robert Waterston, who led the Washington University team.

LAT-WP